1. Field of the Invention
The present invention relates to a network system and a communication method of performing communication via an NAT device using communication devices connected via a network.
2. Description of the Related Art
Upon communication being performed on the Internet, an Internet protocol (hereinafter, referred to as the IP) is used as a routing protocol. Since a currently common IP version 4 uses a “32-bits” number for specifying an address, it has become difficult to specify a unique address to each communication terminal as the Internet becomes popularized. To solve the problem, a method is developed to use a private address assigned to a predetermined limited area (local area), aside from a global address as a global IP address.
In communication between the communication devices using the private address, the private address is translated into the global address by a network address translation table (hereinafter, refereed to as the NAT). Therefore, one private address needs to have one global address corresponding thereto, so that, when a plurality of communication devices are present within one local area, only one communication device within the local area is connected to the Internet. That is, there was a problem that the plurality of communication devices within the same local area had been unable to be connected simultaneously to the Internet. Consequently, a port number is used as a sub-address to establish the simultaneous connection between the plurality of communication devices within the same local area and the Internet.
However, especially with a P2P application, it can not get the private address and the corresponding port number before NAT translation of the communication device on the other side, causing a so-called NAT problem where the P2P application fails. Conventionally, the NAT problem is solved by, for example, providing an NAT device as a gateway from the local area to monitor a communication session that potentially causes the NAT problem and update the addresses and the port numbers in packets on both sides. However, this method is not preferable in that, when a new protocol is developed, it is required to replace the NAT device with a new one compliant with the new protocol, thereby the P2P application may not work correctly until the new NAT device compliant with the new protocol is developed. Moreover, there is another problem in view of the security because the NAT device updates the packet without permission.
Consequently, as means of solving the foregoing NAT problem, methods have been developed to functionalize the P2P application using an UPnP (Universal Plug and Play internet Gateway Device) standard or an STUN (Simple Traversal UDP through NAT) standard.